Abstract
We present a study of the incidence of active galactic nucleus (AGN) in a sample of major merging systems at 0.3 <
z
< 2.5. Galaxies in this merger sample have projected separations between ...3 and 15 kpc and are selected from the CANDELS/3D-Hubble Space Telescope catalogs using a peak-finding algorithm. AGNs in mergers and non-mergers are identified on the basis of their X-ray emission, optical lines, mid-infrared colors, and radio emission. Among galaxies with adequate measurements to find potential AGNs, we find a similar fraction of AGNs in mergers (16.4
%) compared to the fraction found in non-merging galaxies (15.4% ± 0.6%). In mergers, this fraction is obtained by assuming that, in unresolved observations, only one of the merging galaxies is the AGN source (16 merging systems have at least one AGN galaxy). The similarity between the fractions is possibly due to the higher availability of cold gas at high redshifts, where the excess of nuclear activity as a result of merging is less important than at lower redshifts. Star-forming galaxies have a higher incidence of AGNs than quiescent galaxies. In particular, starbursts in mergers are the most common sites of AGN activity, since they present higher AGN fractions and black hole accretion rates. We find no clear correlation between the black hole accretion rate and the galaxy properties (i.e., star formation rate, stellar mass) in mergers and non-mergers. However, mergers seem to have a higher correlation with star formation than non-mergers, which possibly indicates that the merging process is starting to influence the star formation and AGN activity even at this pre-coalescence stage.
Abstract
Observations of low-order
12
C
16
O transitions represent the most direct way to study galaxies’ cold molecular gas, the fuel of star formation. Here we present the first detection of CO(
J
...= 2 → 1) in a galaxy lying on the main-sequence of star-forming galaxies at
z
> 6. Our target, G09-83808 at
z
= 6.03, has a short depletion timescale of
τ
dep
≈ 50 Myr and a relatively low gas fraction of
M
gas
/
M
⋆
≈ 0.30 that contrasts with those measured for lower-redshift main-sequence galaxies. We conclude that this galaxy is undergoing a starburst episode with a high star formation efficiency that might be the result of gas compression within its compact rotating disk. Its starburst-like nature is further supported by its high star formation rate surface density, thus favoring the use of the Kennicutt–Schmidt relation as a more precise diagnostic diagram. Without further significant gas accretion, this galaxy would become a compact, massive quiescent galaxy at
z
∼ 5.5. In addition, we find that the calibration for estimating interstellar medium masses from dust continuum emission satisfactorily reproduces the gas mass derived from the CO(2 → 1) transition (within a factor of ∼2). This is in line with previous studies claiming a small redshift evolution in the gas-to-dust ratio of massive, metal-rich galaxies. In the absence of gravitational amplification, this detection would have required of order 1000 hr of observing time. The detection of cold molecular gas in unlensed star-forming galaxies at high redshifts is thus prohibitive with current facilities and requires a tenfold improvement in sensitivity, such as that envisaged for the Next-Generation Very Large Array .
Abstract
The Antennae Galaxies, one of major mergers, are a starburst. Tsuge et al. (2020, PASJ, 73, S35) showed that the five giant molecular complexes in the Antennae Galaxies have signatures of ...cloud–cloud collisions based on the Atacama Large Millimeter Array (ALMA) archival data with 60 pc resolution. In the present work we analyzed the new CO data toward the super star cluster (SSC) B1 with 14 pc resolution obtained with ALMA, and confirm that two clouds show a complementary distribution with a displacement of ∼70 pc as well as connecting bridge features between them. The complementary distribution shows a good correspondence with the theoretical collision model (Takahira et al. 2014, ApJ, 792, 63), and the distribution indicates that the formation of SSC B1 with ∼106 M⊙ was consistent with the trigger of cloud–cloud collision on a time scale of ∼1 Myr, which is consistent with the cluster age. It is likely that SSC B1 was formed from molecular gas of ∼107 M⊙ with a star formation efficiency of $\sim\! 10\%$ in 1 Myr. We identify a few places where additional clusters are forming. Detailed gas motion indicates that the stellar feedback in the accelerating gas is not effective, while the ionization plays a role in evacuating the gas around the clusters at a ∼20 pc radius. The results have revealed the details of the parent gas where a cluster having a mass similar to a globular is being formed.
We report ∼2″ resolution Atacama Large Millimeter/submillimeter Array observations of the HCN (1-0), HCO+ (1-0), CO (1-0), CO (2-1), and CO (3-2) lines toward the nearby merging double-nucleus galaxy ...NGC 3256. We find that the high-density gas outflow traced in HCN (1-0) and HCO+ (1-0) emission is colocated with the diffuse molecular outflow emanating from the southern nucleus, where a low-luminosity active galactic nucleus (AGN) is believed to be the dominant source of the far-infrared luminosity. On the other hand, the same lines were undetected in the outflow region associated with the northern nucleus, whose primary heating source is likely related to starburst activity without obvious signs of an AGN. Both the HCO+ (1-0)/CO (1-0) line ratio (i.e., dense gas fraction) and the CO (3-2)/CO (1-0) line ratio are larger in the southern outflow (0.20 0.04 and 1.3 0.2, respectively) than in the southern nucleus (0.08 0.01 and 0.7 0.1, respectively). By investigating these line ratios for each velocity component in the southern outflow, we find that the dense gas fraction increases and the CO (3-2)/CO (1-0) line ratio decreases toward the largest velocity offset. This suggests the existence of a two-phase (diffuse and clumpy) outflow. One possible scenario to produce such a two-phase outflow is an interaction between the jet and the interstellar medium, which possibly triggers shocks and/or star formation associated with the outflow.
A galaxy-galaxy merger and the subsequent triggering of starburst activity are fundamental processes linked to the morphological transformation of galaxies and the evolution of star formation across ...the history of the universe. Both nuclear and disk-wide starbursts are assumed to occur during the merger process. However, quantifying both nuclear and disk-wide star formation activity is nontrivial because the nuclear starburst is dusty in the most active merging starburst galaxies. This paper presents a new approach to this problem: combining hydrogen recombination lines in optical, millimeter, and free-free emission. Using NGC 3256 as a case study, Hβ, H40 , and free-free emissions are investigated using the Multi Unit Spectroscopic Explorer at the Very Large Telescope of the European Southern Observatory (MUSE) and the Atacama Large Millimeter/submillimeter Array (ALMA). The Hβ image obtained by MUSE identifies star-forming regions outside the nuclear regions, suggesting a disk-wide starburst. In contrast, the H40 image obtained by ALMA identifies a nuclear starburst where optical lines are undetected due to dust extinction (AV ∼ 25). Combining both MUSE and ALMA observations, we conclude that the total star formation rate (SFR) is 49 2 M yr−1 and the contributions from nuclear and disk-wide starbursts are ∼34% and ∼66%, respectively. This suggests the dominance of disk-wide star formation in NGC 3256. In addition, pixel-by-pixel analyses for disk-wide star-forming regions suggest that shock gas tracers (e.g., CH3OH) are enhanced where gas depletion time (τgas = Mgas/SFR) is long. This possibly means that merger-induced shocks regulate disk-wide star formation activities.
We present results from a deep 2′ × 3′ (comoving scale of 3.7 Mpc × 5.5 Mpc at z = 3) survey at 1.1 mm, taken with the Atacama Large Millimeter/submillimeter Array (ALMA) in the SSA22 field. We ...observe the core region of a z = 3.09 protocluster, achieving a typical rms sensitivity of 60 Jy beam−1 at a spatial resolution of 0 7. We detect 18 robust ALMA sources at a signal-to-noise ratio (S/N) > 5. Comparison between the ALMA map and a 1.1 mm map, taken with the AzTEC camera on the Atacama Submillimeter Telescope Experiment (ASTE), indicates that three submillimeter sources discovered by the AzTEC/ASTE survey are resolved into eight individual submillimeter galaxies (SMGs) by ALMA. At least 10 of our 18 ALMA SMGs have spectroscopic redshifts of z 3.09, placing them in the protocluster. This shows that a number of dusty starburst galaxies are forming simultaneously in the core of the protocluster. The nine brightest ALMA SMGs with S/N > 10 have a median intrinsic angular size of ( physical kpc at z = 3.09), which is consistent with previous size measurements of SMGs in other fields. As expected, the source counts show a possible excess compared to the counts in the general fields at S1.1mm ≥ 1.0 mJy, due to the protocluster. Our contiguous mm mapping highlights the importance of large-scale structures on the formation of dusty starburst galaxies.
Abstract
We present ALMA C ii line and far-infrared (FIR) continuum observations of three $z \gt 6$ low-luminosity quasars ($M_{\rm 1450} \gt -25$ mag) discovered by our Subaru Hyper Suprime-Cam ...(HSC) survey. The C ii line was detected in all three targets with luminosities of $(2.4\mbox{--}9.5) \times 10^8\, L_{\odot }$, about one order of magnitude smaller than optically luminous ($M_{\rm 1450} \lesssim -25$ mag) quasars. The FIR continuum luminosities range from $\lt 9 \times 10^{10}\, L_{\odot }$ (3 $\sigma$ limit) to ${\sim } 2 \times 10^{12}\, L_{\odot }$, indicating a wide range in star formation rates in these galaxies. Most of the HSC quasars studied thus far show C ii/ FIR luminosity ratios similar to local star-forming galaxies. Using the C ii-based dynamical mass ($M_{\rm dyn}$) as a surrogate for bulge stellar mass ($M_{\rm\, bulge}$), we find that a significant fraction of low-luminosity quasars are located on or even below the local $M_{\rm\, BH}$–$M_{\rm\, bulge}$ relation, particularly at the massive end of the galaxy mass distribution. In contrast, previous studies of optically luminous quasars have found that black holes are overmassive relative to the local relation. Given the low luminosities of our targets, we are exploring the nature of the early co-evolution of supermassive black holes and their hosts in a less biased way. Almost all of the quasars presented in this work are growing their black hole mass at a much higher pace at $z \sim 6$ than the parallel growth model, in which supermassive black holes and their hosts grow simultaneously to match the local $M_{\rm\, BH}$–$M_{\rm\, bulge}$ relation at all redshifts. As the low-luminosity quasars appear to realize the local co-evolutionary relation even at $z \sim 6$, they should have experienced vigorous starbursts prior to the currently observed quasar phase to catch up with the relation.
We present the results from Atacama Large Millimeter/submillimeter Array observations of N ii 205 m, C ii 158 m, and O iii 88 m lines in an unlensed submillimeter galaxy at z = 4.3, COSMOS-AzTEC-1, ...hosting a compact starburst core with an effective radius of ∼1 kpc. The C ii and N ii emission are spatially resolved in 0 3-resolution (1 kpc in radius). The kinematic properties of the N ii emission are consistent with those of the CO(4-3) and C ii emission, suggesting that the ionized gas feels the same gravitational potential as the associated molecular gas and photodissociation regions (PDRs). On the other hand, the spatial extent is different among the lines and dust continuum: the C ii emitting gas is the most extended and the dust is the most compact, leading to a difference of the physical conditions in the interstellar medium. We derive the incident far-ultraviolet flux and the hydrogen gas density through PDR modeling by properly subtracting the contribution of ionized gas to the total C ii emission. The observed C ii emission is likely produced by dense PDRs with cm−3 and G0 = 103.5-3.75 in the central 1 kpc region and cm−3 and G0 = 103.25-3.5 in the central 3 kpc region. We have also successfully measured the line ratio of O iii/N ii in the central 3 kpc region of COSMOS-AzTEC-1 at z = 4.3, which is the highest redshift where both nitrogen and oxygen lines are detected. Under the most likely physical conditions, the measured luminosity ratio of LO iii/LN ii = 6.4 2.2 indicates a near solar metallicity with Zgas = 0.7-1.0 Z , suggesting a chemically evolved system at z = 4.3.
We report the detailed modeling of the mass profile of a z = 0.2999 massive elliptical galaxy using 30 milli-arcsecond resolution 1 mm Atacama Large Millimeter/submillimeter Array (ALMA) images of ...the galaxy–galaxy lensing system SDP.81. The detailed morphology of the lensed multiple images of the z = 3.042 infrared-luminous galaxy, which is found to consist of tens of ≲ 100 pc-sized star-forming clumps embedded in a ∼ 2 kpc disk, are well reproduced by a lensing galaxy modeled by an isothermal ellipsoid with a 400 pc core. The core radius is consistent with that of the visible stellar light, and the mass-to-light ratio of
${\sim}\,2\,M_{\odot }\,L_{\odot }^{-1}$
is comparable to the locally measured value, suggesting that the inner 1 kpc region is dominated by luminous matter. The position of the predicted mass centroid is consistent to within ≃ 30 mas with a non-thermal source detected with ALMA, which likely traces an active galactic nucleus of the foreground elliptical galaxy. While the black hole mass and the core radius of the elliptical galaxy are degenerate, a point source mass of > 3 × 108 M
⊙ mimicking a supermassive black hole is required to explain the non-detection of a central image of the background galaxy. The required mass is consistent with the prediction from the well-known correlation between black hole mass and host velocity dispersion. Our analysis demonstrates the power of high resolution imaging of strong gravitational lensing for studying the innermost mass profile and the central supermassive black hole of distant elliptical galaxies.
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